The present invention relates to a monitor circuit for monitoring signal quality of spare channels of digital radio.
In a prior art spare channel monitor circuit which is shown and described in European Patent Application No. 0,243,938 (published Nov. 4, 1987), a single clock recovery circuit is provided for a plurality of digital radio spare channels to recover clock pulses from one of the spare channels. The clock pulses occur at the frequency of AMI (alternate mark inversion) test signals transmitted through the spare channels. The AMI test signals are decoded to recover the original pseudo-random sequence from each spare channel. Signal quality of the spare channels is determined by the occurrences of a mismatch between the decoded signals of the spare channels and this mismatch detection is timed with the clock pulse. However, since pulse stuffing techniques are usually employed in the digital radio transmission channels, "destuffing" jitter occurs in the channels and the timing margin of the mismatch detection must be restricted to avoid the destuffing jitter.